I am considering hiking 100 Mile Wilderness in Maine with my son. He would be 7 at the time. My biggest concern as we are thinking about this are the river fords since some of them can come up to the waist of adults. I think some of the fords would be swims for him (which I am not comfortable with). Can the fords in the 100 Mile Wilderness be bypassed?
I don't think you need to bypass them at all. You didn't say what time of year you'd be going, but in the later summer/fall, they are not nearly that deep.
Also, at any time of year you could easily carry your pack(s) across, then go back and piggy-back your son with a 2nd trip. The first trip would also give you a feel for how rough the crossing would be.
In June 2015 I did the 100 Mile Wilderness (along with the whole AT south bound). Even at that time, with the rivers running hard and sometimes waist deep, it would be quite reasonable to cross in the manner I described.